An aqueous acetic acid solution discharged from the process for the preparation of cellulose acetate contains metallic salts of sulfuric acid, such as Na, K, Ca, Mg, Fe, Al, or Zn sulfate formed by the neutralization of sulfuric acid used as catalyst in the acetylation process (refer to the Japanese Patent Publication No. 17580/1973). Magnesium sulfate, for example, is contained in a concentration not higher than the saturation solubility. Acetic acid has been recovered from this aqueous solution by extraction with ethyl acetate or other organic solvents. In this method, the extraction is not affected by the metallic salts of sulfuric acid contained in the aqueous acetic acid solution, but the method in which an organic solvent such as ethyl acetate is used as an extractant is not fully satisfactory from the viewpoint of the extraction efficiency or energy consumption, because the distribution coefficient of acetic acid is small in general. The efficiency of recovery by extraction is much dependent on the distribution coefficient, and a large amount of solvent must be used to raise the extraction ratio, which results disadvantageously in large energy consumption in the process of separation.
The inventors proposed a process for the extraction of acetic acid from an aqueous solution using an organic solvent containing a tertiary amine as an extractant (refer to the Japanese Patent Laid-Open No. 10131/1981) which is an excellent extractant for the extraction of acetic acid. As a result of further studies of the inventors, however, it was found that in case an organic extractant containing a tertiary amine is used for an aqueous acetic acid solution containing a metallic salt of sulfuric acid, an amine salt of acetic acid reacts with the metallic salt of sulfuric acid in an extractor so that acetic acid is dissolved in the form of a metallic salt in the raffinate, and the extraction efficiency is lowered because of this unrecovered acetic acid.